This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to improving servo performance of a disc drive through the use of different pulse stream combinations to compensate for head construction and performance variations.
Disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, user data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to a host computer. A closed loop digital servo system provide head positional control using servo data written to the discs during manufacturing.
The servo system carries out two primary operations: seeking and track following. A seek involves moving a selected head from track to track, while track following involves causing the head to follow a particular track. It will be noted that data reading and writing operations generally occur while the disc drive is in the track following mode.
A typical seek operation uses a velocity controlled approach wherein a velocity profile, indicative of a desired velocity trajectory for the head, is selected based on the distance between an initial track and a desired, destination track. For a seek of sufficient length, the head is first accelerated to a maximum velocity, maintained at this maximum velocity until it reaches a predetermined distance from the destination track, and then decelerated to settle onto the destination track. Adjustments in the velocity of the head are repetitively made during the seek in relation to the difference between the actual velocity and the desired velocity trajectory of the velocity profile.
Since the various desired velocity values that make up the velocity profile are applied in relation to the number of tracks to go to the destination track, it is necessary for the servo system to keep track of the position of the head as the head crosses each of the intermediate tracks during the seek. This is typically carried out by tranducing the servo data to identify each track crossing as the head is moved toward the destination track. Should the servo system become unable to correctly detect the servo data, the servo system will abort the seek since closed-loop head positional control will be lost at that point.
As will be recognized, improved data transfer performance levels have been achieved in recent generations of disc drives through the introduction of magneto-resistive (MR) heads. An MR head incorporates separate write and read elements, with the write element comprising an inductive coil about a core with a write gap and the read element comprising a magneto-resistive material having a changed electrical resistance in the presence of a magnetic field of selected orientation.
The increased sensitivity of the MR element allows write pulses from the head to be relatively narrow, facilitating higher data transfer rates. However, due to the complexity and minute dimensions of MR heads, it is becoming increasingly difficult to design and manufacture heads that exhibit flawless operation. While manufacturing screening operations attempt to sort flawed heads and remove such from the manufacturing process, some heads nevertheless exhibit intermittent discrepant operation, which is usually manifested as temporary distortion in the readback signal.
Ordinarily, a distorted data readback signal may be corrected with appropriate error correction code (ECC) based techniques. However, such ECC techniques cannot be applied to the position feedback-based servo data. Thus, while MR heads have provided significant gains in data transfer performance, such heads can cause intermittent degradations in servo performance which are not easily correctable. It has been found that some MR heads exhibit a greater tendency to output distorted readback signals upon initialization, before steady-state thermal equilibrium conditions within the heads have been are achieved. Thus, distortion can often occur when the disc drive switches between a first xe2x80x9cwarmxe2x80x9d head to a second xe2x80x9ccoolxe2x80x9d head and immediately attempts to execute a seek operation with the second head.
Accordingly, there is a need for improvements in the art to enable disc drives to correctly decode servo data in the presence of signal distortion from an MR head. It is to such improvements that the present invention is directed.
The present invention provides an apparatus and method for improving disc drive servo control performance.
In accordance with preferred embodiments, a disc drive includes a head adjacent a rotatable disc having a surface on which a plurality of tracks are defined by servo position data. A seek is carried out to move the head from an initial track to a destination track using a velocity controlled approach wherein currents are applied to an actuator motor in relation to distance to go to the destination track.
As the head is moved toward the destination track, a servo circuit decodes servo position data stored on intermediary tracks between the initial track and the destination track by transducing an analog servo readback signal from the servo position data, the servo readback signal having successive pairs of positive and negative peaks. A positive digital pulse stream is generated with pulses indicative of positive amplitude peaks in the servo readback signal. Similarly, a negative digital pulse stream is generated with pulses indicative of negative amplitude peaks in the servo readback signal. The positive and negative digital pulse streams are thereafter combined using a first Boolean logical operator to form a first output digital pulse stream nominally indicative of position of the head.
When the servo circuit determines that first output digital pulse stream fails to correctly indicate the position of the head, typically as a result of distortion in the servo readback signal, the servo circuit selects a second Boolean logical operator to form a second output digital pulse stream from the positive and negative digital pulse streams. While different combinations of Boolean operators can be used to obtain similar results, preferably, a selected one of the Boolean logical operators is an AND operator and the remaining Boolean logical operator is an OR operator.
An AND operator typically provides improved results in the presence of spurious peaks (drop ins) in the servo readback signal caused by, for example, baseline shifts. An OR operator typically provides improved results when peaks are missing (drop outs) from the digital pulse streams, caused by, for example, asymmetry in the servo readback signal.
Preferably, separate logical mode values are stored in a table by head and updated as required during successive seeks for each head. In this way, the servo data are decoded in relation to the logical operator that has most recently been found to provide better servo performance.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.